Conventionally, a fuel vapor leak detection device, which detects a leakage of fuel vapor generated in a fuel tank, is known. The fuel vapor leak detection device detects a leakage of fuel vapor from the fuel tank by changing a pressure in the fuel tank by a pump, which can pressurize and depressurize. The fuel vapor leak detection device is connected to a canister which recovers fuel vapor in the fuel tank. Fuel vapor recovered by the canister is transferred to an intake passage by utilizing air introduced from the atmosphere. The fuel tank connected to the canister communicates with the atmosphere through the canister. Patent Document 1 (JP-A-2004-011561 corresponding to US 2003/0226549 A1) describes regarding a fuel vapor leak detection device of a fuel vapor treatment system, in which a fuel tank is isolated from the atmosphere in a normal state by providing an open/close valve, which opens and closes the fuel tank, in a passage between the fuel tank and a canister.
However, the fuel vapor treatment system described in Patent Document 1 further includes a switching valve besides the open/close valve. The switching valve switches from a connection between the canister and the atmosphere to a disconnection therebetween when detecting a leakage of fuel vapor. Therefore, a volume of the fuel vapor treatment system described in Patent Document 1 is large because the fuel vapor treatment system includes both of the open/close valve and the switching valve.